The invention relates to a delay simulator for FM-CW range measuring apparatus, such as for measuring altitude, or for a radar. The apparatus effects, during real time operation, a frequency comparison between the transmitted wave and the wave received after reflection from a target or retransmission by a responder in such manner as to generate a beat signal Fb of frequency fb. The simulator is intended for connection to the apparatus for testing its proper operation.
The invention relates to testing range measuring apparatus, more particularly to radio altimeters. By way of preferred example, the focus of interest will be for ranges which may vary from 0 to 16 kms (0 to 50,000 feet) and more, and which are to be measured by means of radio altimeters or FM-CW radar devices operating in a frequency band covering some hundreds of MHz and preferably being centered around several GHz, for example the 4.2 to 4.4 GHz band or centered around a lower frequency, for example on the order of 1 GHz. A radio altimeter of this type, utilizing other operating frequencies, is described in French Patent Specification No. 1,557,670 corresponding to U.S. Pat. No. 3,588,899. To put it more precisely, the wish is to be able to test the operation of a high altitude radio altimeter or of a FM-CW radar for ranges up to approximately 16,000 meters.
Before a radio altimeter is made operative, it is necessary to test it, more specifically to check its precision. In the known technique, delay lines of which the time delay .tau..sub.s is known very accurately are used for this purpose. Each delay line is representative of a simulated distance D.sub.s defined by the formula: ##EQU1## c being the speed of an electromagnetic wave in the air. D.sub.s represents the distance the FM-CW apparatus must indicate with a very small error if it operates correctly, when the delay line producing the time delay .tau..sub.s is utilized. When a delay line is connected between the aerials of a radio altimeter, it delays the wave transmitted by the latter by a time .tau..sub.s and, for the radio altimeter, everything occurs as if the wave were transmitted in the air, reflected from an obstacle situated at a distance D.sub.s from the radio altimeter and received by the receiving aerial after the wave has returned through the same distance D.sub.s. (In this comparison, as regards the wave, interest only goes towards the frequency functions as a function of time, it being assumed that the problems of attenuation of the signal between the transmitting and receiving aerials have been resolved). This method of testing a radio altimeter is very accurate but, because of price and bulk of the delay lines required, it is not possible to use an appropriate number for testing the precision at many altitudes (distances). Consequently, in practice this precision is only tested for some points, always the same, and there is a risk that only faults are detected which occur in a limited number of altitude zones. On the other hand, because of the above-mentioned price and bulk reasons, the tests are in practice only effected in a laboratory, whilst it would be desirable to have the possibility to use an air-borne test system.